Geol. Mag. 155 2 , 2018, pp. 316–334. doi:10.1017 ......with the Upper Cambrian Mila Formation of...

Geol. Mag. 155 (2 ), 2018, pp. 316–334. c© Cambridge University Press 2017 316doi:10.1017/S0016756817000218

Contrasting subduction–exhumation paths in the blueschists of theAnarak Metamorphic Complex (Central Iran)

S . Z A N C H E T TA ∗†, N . M A L A S P I NA ∗, A . Z A N C H I ∗, L . B E N C I O L I N I ‡,S . M A RT I N § , H . R . JAVA D I ¶ & M . KO U H P E Y M A ¶

∗Dipartimento di Scienze dell’Ambiente e della Terra, Piazza della Scienza 4, 20126 Milano, Italy‡Dipartimento di Fisica, Chimica e Ambiente, Università degli Studi di Udine, Italy

§Dipartimento di Geoscienze, Università degli Studi di Padova, Italy¶Geological Survey of Iran, Azadi Square, Meraj Avenue, 13185-1494 Tehran, Iran

(Received 2 November 2016; accepted 17 February 2017; first published online 3 April 2017)

Abstract – The Anarak Metamorphic Complex, localized in Central Iran, is a fossil accretionarywedge composed of several tectonometamorphic units. Some of these, the Chah Gorbeh, the Morghaband the Ophiolitic complexes, contain mafic rocks that have been metamorphosed at high-pressure–low-temperature conditions. Such units have been stacked together and later refolded during the fi-nal stages of exhumation. Structural analysis at the mesoscale recognized at least three deforma-tion events. Microstructural analyses, mineral chemistry and thermodynamic modelling reveal thatthe mafic schists followed contrasting P–T paths during their tectonometamorphic evolutions. In theschists of the Chah Gorbeh and Ophiolitic complexes an early greenschist-facies stage was lateroverprinted by blueschist-facies phase assemblages with suggested peak conditions of 390–440 °Cat 0.6–0.9 GPa for the meta-basalt within the Ophiolitic Complex and 320–380 °C at 0.6–0.9 GPafor the blueschists of the Chah Gorbeh Complex. P–T conditions at metamorphic peak were 410–450 °C at 0.78–0.9 GPa for the Morghab blueschists, but they are reached before a greenschist-faciesre-equilibration. Compositional zoning of amphiboles and epidotes of this greenschist-facies stagesuggests a renewed pressure increase at the end of this metamorphic stage. Based on these data wereconstructed a clockwise P–T path for the Morghab mafic schists and a counter-clockwise path forthe Chah Gorbeh blueschists and ophiolitic meta-basalts. Such contrasting metamorphic evolutionsof tectonic units that were later accreted to the same wedge are indicative of the complex tectonicdynamics that occur within accretionary–subduction complexes.

Keywords: Central Iran, mafic blueschists, accretionary wedge, P–T evolution

1. Introduction

The occurrence of mafic blueschists is considered oneof the main points of evidence for oceanic subduc-tion accompanied by the formation of an accretion-ary prism between the upper and the subducting plate(e.g. Ernst, 1973). Blueschist-facies rocks are mainlyfound to occur as large coherent terranes or as isolatedblocks within a serpentinitic or pelitic matrix in tec-tonic mélange complexes (Hsu, 1968).

The P–T conditions at metamorphic peak and thesubduction–exhumation paths followed by blueschist-facies rocks strongly depend on the thermal regimeduring subduction, which in turn depends on the ageand structure of the subducting plate (e.g. Cloos &Shreve, 1988a,b), and on the time at which the rocksare subducted with respect to subduction initiation (i.e.initial versus late-subducted blueschists). Depressedthermal regimes usually develop only after a few mil-lion years following subduction initiation (Gerya &Stockhert, 2006), and rock units that are subductedlater may escape a first heating stage that is commonlyexperienced by units that are underplated in the nas-

†Author for correspondence: [email protected]

cent accretionary wedge at subduction initiation (Ukar,2012).

The common occurrence within a single accretion-ary complex of blueschists that have reached differentdepths and followed different subduction–exhumationpaths (e.g. Agard et al. 2010) testifies to the complex-ity of the tectonic and thermal structure of an evolvingaccretionary wedge.

In Central Iran, the Anarak Metamorphic Complex(AMC) is interpreted as a fossil accretionary prism(Sharkovski, Susov & Krivyakin, 1984; Bagheri &Stampfli, 2008) developed in the framework of thenorthward subduction of the Palaeo-Tethys beneaththe southern Eurasian margin (e.g. Şengör, 1979). TheAMC consists of several units, some of which weremetamorphosed at blueschist-facies conditions (Zan-chi et al. 2009, 2015). The difference in lithologicalcomposition, metamorphic peak conditions and age ofmetamorphism led some authors to consider the AMCas a composite complex, formed during tectonic eventsthat were separated in time and space.

In this paper, we address the petrographic and tex-tural features of mafic blueschists of the AMC, inorder to reveal the recrystallization stages and thedeformation–metamorphism relationships in the Chah

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Blueschists of the Anarak Metamorphic Complex (Iran) 317

Figure 1. (Colour online) Geological map of the Anarak Metamorphic Complex with location of analysed samples. The trace of thegeological cross-section in Figure 2 is here indicated. Modified after Zanchi et al. (2015).

Gorbeh, Morghab and Ophiolitic complexes. We alsoperformed thermodynamic modelling in order to es-timate the P and T conditions of each metamorphicstage recognized in the analysed samples.

The whole dataset of petrographic, microstructuraland thermodynamic data is then used to define theP–T trajectories of mafic blueschists from the threeunits. The results are then discussed in the contextof accretionary wedge dynamics and AMC tectonicsignificance.

2. Geological setting

The AMC crops out in Central Iran, to the east ofNain (Fig. 1). It consists of several subunits exposedfor c. 150 km in an E–W-trending belt, stacked to-gether during polyphase metamorphic and deforma-tion events (Sharkovski, Susov & Krivyakin, 1984; Ba-gheri & Stampfli, 2008; Zanchi et al. 2009; Buchset al. 2013; Zanchi et al. 2015). The AMC is in tec-tonic contact with other metamorphic complexes andsedimentary successions of various ages and palaeo-geographic affinities (Fig. 1). To the west, the GreatKavir – Doruneh fault system represents the contact

between the AMC and the Cretaceous ophiolites thatborder the entire Central–East Iranian Microcontin-ent (Ghasemi & Talbot, 2006). The southern limitof the AMC coincides with the NW border of theYadz block whereas to the east the Jandaq complex, amagmatic–metamorphic unit with a poorly defined age(likely Carboniferous or pre-Carboniferous, Bagheri &Stampfli, 2008; Berra et al. 2017), is exposed. Finally,to the north of the AMC, the non-metamorphic Na-khlak ophiolite–sedimentary complex occurs (Baliniet al. 2009). The contact between the AMC and theNakhlak complex is not exposed, so the relationshipsexisting between the complexes are not clear (Baliniet al. 2009; Zanchi et al. 2009).

The AMC is composed of several subunitswhich display heterogeneous structural and meta-morphic evolutions (Sharkovski, Susov & Krivyakin,1984; Bagheri & Stampfli, 2008; Zanchi et al.2009, 2015). The Morghab Complex largely con-sists of metapelitic schists with abundant interleavedquartzite and metabasite layers (Zanchi et al. 2015).Minor meta-volcanic rocks and amphibole-bearinggneisses occur in the Kuh-e Pol-e Khavand area(Fig. 1).


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318 S . Z A N C H E T TA A N D OT H E R S

Figure 2. (Colour online) Schematic cross-section showing the main structural features of the AMC and the tectonic relationshipsbetween the CGC, MC and OC complexes and the overlying Lakh Marble. Modified after Zanchi et al. (2015).

We will focus in this paper on the three main unitsthat compose the AMC: the Chah Gorbeh (CGC),Morghab (MC) and Ophiolitic (OC) complexes (Zan-chi et al. 2015).

Quartzite-rich phyllites, micaschists and metaba-sites interlayered with thick metacarbonate layers(Figs 1, 2) form the CGC. Serpentinite lenses and lay-ers occur in the southern part of the CGC.

The MC occurs to the north and east of theCGC. The MC has a more monotonous lithologicalcomposition, chiefly consisting of metapelites (phyl-lites and micaschists) with minor intercalations ofquartzites, metabasites and thin marble layers. Re-gional folds with steep fold axes affected both theMC and CGC; this feature is particularly evident atthe eastern end of the CGC (Fig. 1). To the southand southeast of the CGC an ‘ophiolitic’ complex(OC) occurs (Bagheri & Stampfli, 2008; Zanchi et al.2015). This complex largely consists of serpentin-ites and partly serpentinized harzburgites, where rel-ics of the primary mineral assemblages and tex-tures are scarcely preserved. In the central area ofthe OC some metabasite lenses have been observed(Fig. 1). Such lenses consist of metamorphosed basaltswith well-preserved pillow structures. The OC is in-tensively folded with minor shear zones localizedboth along lithological contacts as well as withinserpentinites.

The CGC, MC and OC units of the AMC are over-thrust by a large thrust sheet of metacarbonates, theLakh Marble (Bagheri & Stampfli, 2008; Zanchi et al.2015). These strongly recrystallized carbonates dis-play an uneven distribution of deformation, with iso-clinally folded areas coexisting with more preservedparts with sedimentary structures such as bedding, in-traformational paraconglomerates (i.e. rich in matrix)and fossils. The age of the Lakh Marble is substantiallyunknown, although a late Palaeozoic age has been pro-posed based on indirect evidence (Bagheri & Stampfli,2008). Recent re-examination of the presumed LowerCambrian Archeocyatid faunas described by Russianauthors in the past (Mel’nikov et al. 1986) resulted inthe identification of the demosponge genus Rankenella(Kruse & Zhuralev, 2008) in the Kabudan Marble,which is a correlative of the Lakh Formation. Theselatter authors thus suggested a possible correlationwith the Upper Cambrian Mila Formation of the Al-borz Mountains, which implies a Gondwanan affinityfor this unit.

Time constraints on the age of the high-pressure–low-temperature (HP–LT) metamorphism of the AMCare still lacking. A minimum age is provided by unde-formed and non-metamorphosed trondhjemitic dykesand stocks that intruded the AMC (Torabi, 2012).Late Permian (Bagheri & Stampfli, 2008) to Sak-marian ages (Zanchi et al. 2015) have been providedfor these intrusive rocks, suggesting that deformationand metamorphism of the AMC is Early Permian orolder.

The AMC is currently interpreted as an alloch-thonous crustal fragment that was part of an accre-tionary wedge developed along the southern Euras-ian margin, in the hangingwall of the Palaeo-Tethyssubduction zone (Zanchi et al. 2015). In this inter-pretation, the formation of the AMC is consideredto have preceded the collision of the Iran platewith Eurasia that led to the Cimmerian orogenicevent.

3. Meso-structural aspects of the AMC

We summarize here the main structural features ofthe metabasite-bearing units of the AMC as observedin the field. For a more exhaustive description of thestructural setting and evolution of the AMC see Zan-chi et al. (2015).

3.a. Morghab Complex

The MC displays a complex structural pattern formedin response to superposed deformation events, the firsttwo out of three associated with syn-metamorphic iso-clinal folding. D1 structures are now preserved at themesoscale only as relicts within the S2 regional fo-liation. Rootless fold hinges are somewhat visible inquartzites and quartz-rich schists, whereas D1-relatedstructures are visible only at the microscale in meta-basites. Mineral phase assemblages in textural equi-librium with D1 structures point to HP–LT conditionsduring D1 (Zanchi et al. 2015).

The D2 deformation event is characterized by iso-clinal folding developed both at the meso- and re-gional scale. The axial plane foliation, S2, of thisfold system is the most pervasive fabric elementwithin all the rock types that form the MC. Be-sides the S2 foliation a well-developed stretchinglineation defined by elongated quartz and chloritehas been frequently observed. The S2 foliation is




Figure 3. (Colour online) (a) Marble layers interleaved with metapelites and mafic schists of the CGC. (b) A fold hinge with a nearlyvertical fold axis in the metacarbonates of the CGC. (c) S1 relict foliation preserved within the regional S2 foliation in paragneiss.Hammer for scale is 33 cm long. (d) S1 relics within schists of the MC. Pen for scale is 14 cm long. (e) Superposed D1–D2 folds inthe MC rocks. Pen for scale is 15 cm long. (f) Open to medium-closed D3 folds with an axial fracture cleavage. Pen for scale is 15 cmlong.

concordant with lithological contacts within the MCand between the MC and adjoining units. Fold axesrelated to D2 folds range from sub-horizontal tonearly vertical, suggesting that a successive deform-ation event affected the complex. This event, D3, isresponsible for the development of medium-tight toclosed folds observable both at the meso- and regionalscale. D3 folds deflecting the S2 foliation and affect-ing the contact between the CGC and MC complexesare clearly reported in the geological-structural mapof Figure 1 and characterize the present-day structuralsetting.

3.b. Chah Gorbeh Complex

As in the MC the oldest recognized structures arepoorly visible at the mesoscale. They are recogniz-able as relict S1 foliation intrafolial in the dominant S2one (Fig. 3c, d). Rootless fold hinges seldom occur inmetacarbonates and metapelites/metapsammites. TheS1 foliation is poorly visible at the mesoscale butclearly evident at the microscale where it is defined bya greenschist-facies mineral assemblage (Fig. 5).

The D2 deformation event formed isoclinal fold-ing associated with a pervasive axial plane foliation,




Figure 4. (Colour online) (a) Field aspect of pillow structure within meta-basalts of the OC; person for scale is c. 1.55–1.6 m tall. (b)Sheared domains developed at the rims of pillow. (c) Mylonitic foliation defined by blueschist-facies minerals at the contact betweenmeta-basalts and serpentinites. (d) Calcite and quartz veins cross-cutting sheared serpentinites. Hammer for scale is 40 cm long.

S2, defined by Na-amphibole (Zanchi et al. 2015) thatpoints to HP–LT metamorphism during the D2 event.The different P–T conditions at which the regional fo-liation formed in the MC (greenschist facies) and CGC(blueschist facies) suggests that, even if the main foli-ations of both units are almost parallel to their contacts,they could not be related to a single deformation eventthat affected the MC and CGC together.

The contact between the CGC and serpentinites ofthe OC is marked by ductile shear zones (Fig. 2),sometimes with lenses of ophicalcites and impuremarbles, where Na-amphibole is stable along themylonitic foliation. This suggests that the MC and ser-pentinites were tectonically coupled at HP–LT con-ditions. In a few localities, in the southern part ofthe CGC, serpentinites are deformed by D2 isoclinalfolds together with rocks of the CGC. This observationsuggests that the OC and serpentinites share a com-mon tectonometamorphic evolution from the D2 eventonwards.

3.c. Ophiolitic Complex

We refer here to the whole complex of ultramafic rocksand blueschist-facies meta-basalts, the same that were

distinguished by Zanchi et al. (2015) into two units:‘ultramafic rocks’ and ‘blueschist pillow meta-basalts’.The contacts between the ultramafic rocks and meta-basalts are always tectonic (Fig. 4c), so a direct correl-ation among structures observed in the two units is notfeasible, justifying the distinction proposed by Zanchiet al. (2015).

Ultramafic rocks occur in two distinct structural set-tings: (1) as tectonic slices within the southern part ofthe CGC, as previously described; (2) as a continu-ous unit, forming complex imbricated thrust sheets,in a lower structural position with respect to the CGC(Fig. 2). Peridotites of type (1) are almost completelyserpentinized and intensively sheared with foliationssubparallel to the S2 of the CGC and to tectonic con-tacts. Peridotites of type (2) show a patchy distribu-tion of deformation with isolated lenses conserving amassive texture with preserved olivine, orthopyroxeneand spinel crystals. This mineral assemblage indic-ates a harzburgitic composition for the ultramafic rocksof the AMC. Such massive domains are wrapped bysheared domains where a pervasive schistosity is as-sociated with an almost complete serpentinization ofharzburgites.




Figure 5. (Colour online) Microstructural features of mafic schists. (a, b) Static growth of blue amphibole (Amp2), magnetite andtitanite (Ttn2) on existing greenschist-facies assemblage in meta-basalts. (c, d) Syn-kinematic Na-amphibole (Amp2) and epidote (Ep2)aligned parallel to S2 within mafic schists of the CGC. Relics of a greenschist-facies S1 foliation are best preserved within plagioclaseporphyroblasts. (e, f) Blueschist-facies phase assemblage aligned defining the internal foliation of plagioclase porphyroblasts (Pl2) inretrogressed blueschists of the MC.

Meta-basalts with well-preserved pillow struc-tures (Fig. 4a, b) occur ESE of Chah Derakhtak(Fig. 1). Such slices form part of a complex S-verging thrust stack also involving serpentinites andophicalcites (Sharkovski, Susov & Krivyakin, 1984;Zanchi et al. 2009, 2015). Except within shearzones, where pillows are partially to almost com-pletely transposed (Fig. 4c), the meta-basalts are sub-stantially undeformed. A poorly defined foliationis distinguishable only at the microscale, definedby greenschist-facies mineral assemblage (see nextsection).

4. Microstructural and metamorphic evolution

Microstructural analyses on selected samples of maficrocks from the three tectonic units were performedto decipher the deformation–metamorphism relation-ships in each unit. The multi-stage structural evol-ution reconstructed at the mesoscale has also beenrecognized at the microscale. A brief overview ofthe chemical composition of key minerals occurringat distinct microstructural positions is provided here,whereas the reader is referred to the next section fora complete description of the AMC rocks’ mineral




chemistry. Mineral abbreviations used in the text arereported in Table 1.

4.a. Morghab Complex

The mineralogy of mafic schists of the MC is madeof (Table 1): amphibole, plagioclase, epidote, chlor-ite, white mica, titanite, quartz, magnetite and apatite.Secondary calcite occurring in veins and microcrackspost-dating the main foliation has been observed insome samples.

Metabasites of the MC are fine grained with themain fabric element at the microscale represented bya well-developed foliation. Cores of epidote (Ep2) andalbitic plagioclase (Pl1) porphyroblasts partially pre-date the main foliation. Aligned inclusion trails havebeen observed both within Pl2 and Ep2. They con-sist of blue Na-rich amphibole (Amp1), white mica(Wm1), titanite (Ttn1), quartz, albite (Pl1) and epi-dote (Ep1). Pl1 and Ep1 inclusions occur within Ep2and Pl2, respectively. The main foliation, which cor-responds to the regional S2 foliation as recognizedby field structural analysis, wraps around plagioclaseand epidote porphyroblast cores. The outer rims ofboth Ep2 and Pl2 seem to overgrow the foliation,so we consider the porphyroblast rims to be syn- topost-kinematic with respect to S2. The S2 foliation isdefined by the shape preferred orientation (SPO) ofAmp2 + Chl + Wm2 + Ttn2. Amp2 occurs in the formof tiny to fine-grained crystals with a subidiomorphicto idiomorphic (prismatic) habit. They frequently con-tain inclusions of quartz, chlorite and plagioclase. Ttn2forms trails of (sub)millimetric (max. 1.5 mm) crys-tals aligned parallel to S2. Ttn2 crystals always have thetypical lozenge-shaped prismatic habit of titanite withthe major axis elongated parallel to the S2 foliation.Both Ttn2 and Amp2 have been observed as inclusionsin Ep2 porphyroblasts rims, suggesting that epidotecrystal growth outlasted the D2 deformation event. Acolour zoning is frequently observed in Amp2: min-erochemical analyses (see next section) highlighted aNa-increase from the core to the rim of Amp2 crystals.A similar compositional zoning also occurs in Ep2 por-phyroblasts, with rims enriched in Fe3+ with respect tothe cores. This suggests a pressure increase during thelate stages of the D2 deformation event. Amp2 crystalsalso occur as inclusions within Ep2 rims.

Summarizing the metamorphic evolution individu-ated for the mafic schists of the MC we candefine three successive parageneses: (i)Blueschist fa-cies (pre-S2): Amp1 + Wm1 + Ttn1 + Ep1 + Pl1 + Qtz;(ii) Greenschist facies (syn-S2): Amp2 (core) + Chl +Wm2 + Ep2 (core) + Pl2 + Ttn2 + Qtz; (iii) Green-schist with P increase (late-, post-S2): Amp2(rim) +Ep2(rim).

4.b. Chah Gorbeh Complex

The mineralogy of metabasites of the CGC consists of:amphibole, plagioclase, chlorite, white mica, epidote, Tab

le1.

Sam

ple

desc

ript

ions

and

loca

tion

s

Min

eral

ogy

Coo

rdin

ates

Sam

ple

Tect

onic

unit

Roc

kty

peQ

tzP

lW

mE

pC

cC

hlR

tT

tnA

mp

Mt

oxA

pZ

rP

yrL

atit

ude

(N)

Lon

gitu

de(E

)

AK

33A

Pil

low

met

a-ba

salt

sB

lues

chis

tx

xx

xx

xx

x33°

23′ 4

1′′

53°

45′ 1

0′′

AK

33B

Pil

low

met

a-ba

salt

sB

lues

chis

tx

xx

xx

x33°

23′ 4

1′′

53°

45′ 1

0′′

AK

36P

illo

wm

eta-

basa

lts

Blu

esch

ist

xx

xx

xx

xx

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6′′

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7′′

I13-

06C

hah

Gor

beh

Blu

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xx

xx

xx

xx

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2′′

53°

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1′′

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09C

hah

Gor

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Blu

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xx

xx

xx

xx

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25′ 5

3′′

53°

44′ 4

8′′

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11C

hah

Gor

beh

Blu

esch

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xx

xx

xx

xx

xx

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25′ 5

1′′

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44′ 4

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14A

Mor

ghab

Gre

ensc

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(Blu

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s)x

xx

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Min

eral

abbr

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tion

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–am

phib

ole;

Ap

–ap

atit

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calc

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–ch

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epid

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–w

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als.




Figure 6. (Colour online) Representative amphibole compositions of mafic schists for the CGC, MC and OC. Arrows indicate theevolution direction (old to young) as recognized by microstructural analyses.

quartz, titanite, rutile, calcite, apatite, zircon and pyrite(Table 1).

The textural features of the mafic schists from theCGC closely resemble the ones described for the rocksof the MC. The development of a primary foliation ispreceded by plagioclase (Pl1) and epidote (Ep1) por-phyroblast growth, with a peculiar mineral phase as-semblage of oriented inclusions. However, the meta-morphic evolution is discordant, as we will describebelow.

An internal foliation (S1) made by the preferredorientation of Ttn1 + Pl1 + Qtz + Amp1 inclusions ishighlighted within Pl1 and Ep1 porphyroblasts. Amp1is actinolitic hornblende (Fig. 6; Table 3), suggestingequilibration at relatively low P under greenschist-facies conditions. The main foliation (S2) is definedby the SPO of Chl + Ab + Wm1 + Ep2 + Rt + Amp2.Blue-coloured Amp2 is ferroglaucophane (Table 3) incomposition and displays clear cores without inclu-sions, coupled with inclusion-rich rims, where Wm1,Chl and Ep2 aligned parallel to S2 occur. Ep2 is Fe-enriched (Fig. 6) with respect to Ep1, pointing to a Pincrease from the D1 to D2 deformation/metamorphicevent. Tiny magnetite crystals occur as trails alignedparallel to S2. Rutile, usually overgrown by rims of ti-tanite (Ttn2), has also been observed only outside Pl1and Ep1 porphyroblasts. Therefore, its growth and in-complete substitution by Ttn2 could be assigned to thesyn- to post-S2 metamorphic stage.

Two main metamorphic stages can be defined forthe CGC: (i) Greenschist facies (pre-S2): Ep1 + Pl1 +Ttn1 + Amp1 + Wm1 + Qtz; (ii) Blueschist facies(syn- to post-S2): Amp2 + Chl + Ep2 + Pl2 + Mt +Wm1 + Rt (Ttn2)

4.c. Ophiolitic Complex (meta-basalts)

As described in Section 3.c, meta-basalts crop outin the southern area of the AMC in tectonic contactwith serpentinized ultramafic rocks (Figs 1, 2). As ob-served in outcrops, where the primary pillow structuresare completely preserved, the meta-basalts also showlittle deformation at the microscale. The complete min-eralogy is defined by: amphibole, plagioclase, chlor-ite, epidote, white mica, magnetite, apatite and Fe–Tioxides.

Meta-basalts are fine grained with a verypoorly defined foliation (S1) made by the SPO ofPl1 + Chl + Amp1 + Ep1 + Ttn1, with Amp1 display-ing an actinolitic composition. This weak foliation isoverprinted by the static growth of prismatic Na-richblue amphibole (Amp2) and aggregates of titanitecrystals (Ttn2) that show no evidence of deformation.Besides their occurrence as relatively large (up to2 mm) prismatic crystals (Fig. 5), Amp2 has also beenobserved to grow as thin coronae around Chl + Amp1sites (Fig. 5). Based on microstructural analysis, themeta-pillow basalts show a prograde metamorphic




evolution, with a greenschist-facies foliation overprin-ted by the static growth of a HP–LT blueschist-faciesphase assemblage as summarized below:

(i) Greenschist (syn-S1): Amp1 + Pl1 + Chl +Wm1 + Ep1 + Ttn1 + Qtz; (ii) Blueschist (post-S1):Amp2 + Ttn2 + Wm2.

5. Mineral chemistry and P–T–d–t paths

5.a. Methods

Textural and mineralogical features were inspectedon polished thin-sections using back-scattered elec-tron (BSE) and secondary electron images and char-acterized by electron microprobe analyses. Analyseswere performed at the Earth Science Department ofthe University of Milano on a Jeol JXA 8200 Super-probe equipped with five wavelength-dispersive spec-trometers (WDS). Analyses were conducted with a c.1 µm beam diameter at conditions of 15 kV and 5 nAof probe current. Natural silicates and oxides wereused as standards. All standards were calibrated within0.5 % at one standard deviation. Raw data were correc-ted using a Phi-Rho-Z quantitative analysis program.Major elements and recalculated structural formulaeare reported in Tables 2, 3 and 4.

Whole-rock analysis for major elements, used forthe isochemical P–T pseudosection (see below), wasdetermined at the ACME Analytical Laboratories Ltdin Vancouver (Canada). Total abundances of the majoroxides were obtained by ICP-ES (inductively coupledplasma emission spectroscopy) following a LiBO2 fu-sion and dilute nitric digestion for major oxides. Ana-lytical errors are within 2 %.

The FeO weight % on total FeO* has been de-termined for the AK14 sample by titration followingthe Wilson method (Wilson, 1955; Yokoyama & Na-kamura, 2002 and references therein) at the Depart-ment of Earth and Environmental Sciences, Universityof Milano Bicocca.

5.b. Results

5.b.1. Morghab Complex

Porphyroblastic epidote and plagioclase pre-serve a relict paragenesis composed byAmp1 + Wm1 + Ttn1 + Ep1 + Pl1 + Qtz (fig. 12bin Zanchi et al. 2015). Micron-sized Amp1 is analkali-amphibole (NaB > 1.50) and corresponds to aglaucophane (Table 2), while Amp2 along the mainfoliation is calcic to sodic-calcic (Ca + Na) B > 1,0.5 < NaB < 1.5 and variable CaB. This mainlycorresponds to actinolite and magnesiokatophoriteand shows a slight zoning from core to rim, with adecrease in CaO, complementary to a slight increasein Na2O (Table 2; Fig. 6d). In addition epidote shows astrong compositional variation. Ep1 included in albiteporphyroblasts show higher FeO* contents with re-spect to the cores of Ep2, which in turn is characterizedby a compositional zoning with the rim enriched in

FeO* with respect to the core (Table 2). This progradezoning is likely related to the core-to-rim increase inNa2O recorded by Amp2. Both white micas, Wm1included in porphyroblastic albite and Wm2 alongthe S2 foliation, show a moderate celadonitic content,in the range of 3.35–3.50 atoms per formula unit(apfu) (Table 2). Finally, plagioclase is mainly albiteand does not show any anorthite substitution in bothgenerations.

5.b.2. Chah Gorbeh Complex

Different from the Morghab mafic rocks, these samplesshow a prograde evolution from greenschist-facies to ablueschist-facies overprint, as recorded also by a dif-ferent mineral composition. The main foliation is infact characterized by rare actinolitic amphibole andabundant alkali-amphiboles (NaB > 1.50) showing ho-mogeneous composition with the highest Na contents(up to 2.03 apfu, Table 3). They are classified as fer-roglaucophane. In a Na v. Ca (Fig. 6a, b) and (Na + K)v. Al(IV) diagram (Fig. 6d, e) they show a trend sim-ilar to that of amphiboles from the MC with the dif-ference that this is prograde. Na and Na + K becomeenriched with respect to Ca and Al(IV), respectively,from Amp1 to Amp2 as indicated by the grey arrow.Glaucophane is characterized by inclusion-rich rims.Such inclusions consist of the same mineral phases asthose crystallized along the main foliation (Chl, Ab,Wm, Ep and rutile with Ttn coronae). In some samples,epidote along the S2 foliation also shows a poikilo-blastic texture and includes quartz, albite and titanite.Such epidotes are enriched in FeO*, in agreement withequilibration with glaucophane (Table 3) and do notshow the strong iron variability as recorded by epi-dotes of the mafic rocks from the MC. Both whitemicas, Wm1 included in glaucophane rims and por-phyroblastic albite, and Wm2 along the S2 foliation,show a moderate celadonitic content, in the range of3.30–3.40 apfu (Table 3). Again, plagioclase is mainlyalbite and does not show any anorthite substitution.

5.b.3 Ophiolitic Complex (meta-basalts)

Meta-basalts from the OC show a very weak foli-ation made of Pl1 + Chl + Amp1 + Ep1 + Ttn1, withAmp1 displaying an actinolitic composition, overprin-ted by the static growth of Amp2 and aggregates ofTtn2. Amp2 occur both as fine-grained aggregates andas millimetre-sized prismatic crystals. As shown inTable 4, fine-grained Amp2 has a variable composi-tion, locally between alkali to sodic-calcic amphiboles(NaB from 1.29 to 1.69 apfu). Coarse Amp2 is in-stead homogeneous in composition and corresponds toa magnesioriebeckite. It shows slightly lower Na con-tent with respect to alkali-amphibole occurring in theChah Gorbeh mafic rocks (Table 4; Fig. 6c, f). Somecoarse white micas appear in microstructural equilib-rium with Amp2 and are characterized by a higherceladonitic substitution, up to 3.55 (Table 4), with



Blueschists

oftheA

narakM

etamorphic

Com

plex(Iran)

325Table 2. Average composition of major elements (ox. wt %) and of recalculated structural formulae of minerals composing mafic rocks from the Morghab Complex (numbers in brackets are standard deviations)

Amphibole Epidote Titanite Plagioclase Phengite

AK14A AK19B AK14A AK19B AK14A AK14A AK19B AK14A AK19B AK19B

Sample MineralMicrostructuralposition

Amp1Inc inEp2

Amp2Core

Amp2Rim

Amp1Inc inEp2

Amp2Core

Amp2Rim

Ep1Inc in

AbEp2Core

Ep2Rim

Ep2Core

Ep2Rim

Ttn1Inc inEp2 Ttn2

AbPorphCore

AbPorphRim Ab2

AbPorph

Wm1Inc in

Ab Wm2

Wm1Inc in

AbChlorite

Chl

No. analyses 9 5 5 7 12 14 2 4 6 4 4 2 4 1 1 1 2 3 8 3 2

SiO2 55.79(0.78)

51.74(1.11)

50.83(0.99)

55.85(0.48)

51.40(1.81)

49.44(1.06)

37.96(0.35)

38.23(0.25)

37.49(0.26)

37.92(0.19)

37.18(0.44)

30.78(0.37)

30.34(0.08)

69.21 68.63 69.33 68.54(0.52)

51.41(0.78)

52.29(1.99)

51.29(1.35)

26.54(0.52)

TiO2 0.03(0.03)

0.50(0.96)

0.06(0.02)

0.03(0.02)

0.07(0.05)

0.12(0.05)

0.06(0.00)

0.05(0.02)

0.04(0.03)

0.06(0.03)

0.04(0.04)

35.36(2.59)

38.03(0.29)

0.01 0.00 0.00 0.00(0.00)

0.07(0.02)

0.14(0.11)

0.07(0.03)

0.08(0.06)

Al2O3 8.58(1.10)

4.02(1.46)

4.55(0.93)

7.99(0.89)

3.75(1.47)

6.44(0.77)

24.47(2.54)

26.55(0.27)

22.63(0.24)

25.23(0.73)

23.20(1.31)

1.88(1.33)

1.01(0.20)

19.88 19.76 20.00 19.74(0.21)

28.44(1.84)

25.65(1.55)

30.47(2.29)

19.62(0.25)

Cr2O3 0.01(0.01)

0.01(0.02)

0.02(0.03)

0.00(0.01)

0.03(0.03)

0.01(0.02)

0.03(0.04)

0.02(0.03)

0.00(0.00)

0.02(0.03)

0.03(0.03)

0.06(0.01)

0.01(0.02)

0.01 0.02 0.02 0.02(0.03)

0.02(0.04)

0.01(0.01)

0.03(0.05)

0.00(0.00)

FeO* 17.82(0.52)

17.06(0.94)

19.41(1.17)

16.91(0.76)

16.17(1.82)

18.44(0.72)

10.68(3.10)

8.61(0.25)

13.25(0.37)

10.26(0.96)

12.69(1.83)

1.17(0.63)

0.47(0.15)

0.01 0.09 0.14 0.09(0.11)

4.04(0.26)

5.20(1.67)

3.02(0.39)

25.92(0.31)

MgO 6.65(0.36)

11.34(0.98)

10.12(0.87)

7.91(0.27)

12.73(1.41)

10.44(0.63)

0.01(0.02)

0.02(0.01)

0.01(0.01)

0.01(0.01)

0.03(0.01)

0.00(0.00)

0.01(0.01)

0.00 0.00 0.01 0.01(0.00)

2.67(1.50)

3.10(0.39)

2.00(0.63)

15.79(0.30)

MnO 0.18(0.05)

0.28(0.02)

0.32(0.04)

0.17(0.05)

0.35(0.06)

0.36(0.03)

0.17(0.09)

0.14(0.04)

0.21(0.04)

0.23(0.08)

0.20(0.08)

0.03(0.02)

0.03(0.03)

0.02 0.00 0.00 0.00(0.00)

0.02(0.02)

0.04(0.04)

0.02(0.03)

0.42(0.09)

CaO 1.66(0.88)

10.01(0.76)

8.73(0.66)

1.00(0.32)

10.32(0.89)

7.90(0.41)

23.03(0.44)

23.37(0.09)

22.56(0.11)

23.16(0.24)

22.75(0.13)

28.02(0.16)

28.19(0.17)

0.06 0.05 0.08 0.05(0.01)

0.09(0.09)

0.07(0.04)

0.01(0.01)

0.04(0.01)

Na2O 6.70(0.28)

1.89(0.44)

2.56(0.50)

6.77(0.17)

1.50(0.57)

3.17(0.26)

0.01(0.01)

0.00(0.00)

0.01(0.01)

0.01(0.00)

0.01(0.02)

0.01(0.02)

0.02(0.02)

11.87 11.96 11.50 11.47(0.04)

0.28(0.05)

0.15(0.05)

0.38(0.10)

0.01(0.01)

K2O 0.02(0.02)

0.17(0.05)

0.21(0.04)

0.01(0.01)

0.15(0.07)

0.22(0.04)

0.00(0.00)

0.00(0.00)

0.00(0.01)

0.00(0.00)

0.00(0.00)

0.01(0.01)

0.02(0.01)

0.04 0.05 0.04 0.05(0.00)

9.43(0.23)

9.74(0.21)

9.78(0.16)

0.01(0.01)

Total 97.45(0.43)

97.01(0.45)

96.81(0.41)

97.36(1.29)

96.56(0.77)

96.80(0.79)

96.42(0.21)

96.99(0.26)

96.21(0.25)

96.90(0.13)

96.13(0.32)

97.32(0.14)

98.14(0.38)

101.11 100.56 101.12 99.96(0.26)

96.47(0.95)

96.40(1.30)

97.06(0.18)

88.40(0.23)

Si 7.94(0.11)

7.59(0.15)

7.54(0.11)

7.98(0.06)

7.56(0.19)

7.34(0.12)

3.02(0.01)

3.00(0.01)

3.01(0.02)

2.99(0.02)

2.98(0.02)

1.02(0.01)

1.00(0.00)

2.99 2.98 3.00 3.00(0.02)

3.39(0.03)

3.47(0.07)

3.34(0.09)

2.79(0.05)

Ti 0.00(0.00)

0.05(0.11)

0.01(0.00)

0.00(0.00)

0.01(0.01)

0.01(0.01)

0.00(0.00)

0.00(0.00)

0.00(0.00)

0.00(0.00)

0.00(0.00)

0.88(0.07)

0.94(0.01)

0.00 0.00 0.00 0.00(0.00)

0.00(0.00)

0.01(0.01)

0.00(0.00)

0.01(0.01)

Al 1.44(0.18)

0.70(0.25)

0.80(0.17)

1.35(0.15)

0.65(0.26)

1.13(0.14)

2.29(0.21)

2.46(0.02)

2.14(0.02)

2.35(0.06)

2.19(0.11)

0.07(0.05)

0.04(0.01)

1.01 1.01 1.02 1.02(0.01)

2.21(0.14)

2.01(0.11)

2.34(0.17)

2.43(0.04)

Cr 0.00(0.00)

0.00(0.00)

0.00(0.00)

0.00(0.00)

0.00(0.00)

0.00(0.00)

0.00(0.00)

0.00(0.00)

0.00(0.00)

0.00(0.00)

0.00(0.00)

0.00(0.00)

0.00(0.00)

0.00 0.00 0.00 0.00(0.00)

0.00(0.00)

0.00(0.00)

0.00(0.00)

0.00(0.00)

Fe* 2.12(0.07)

2.09(0.12)

2.41(0.15)

2.02(0.09)

1.99(0.24)

2.29(0.10)

0.71(0.21)

0.56(0.02)

0.89(0.02)

0.68(0.07)

0.85(0.13)

0.03(0.02)

0.01(0.00)

0.00 0.00 0.00 0.00(0.00)

0.22(0.02)

0.29(0.10)

0.16(0.02)

2.28(0.04)

Mg 1.41(0.08)

2.48(0.20)

2.24(0.18)

1.68(0.06)

2.79(0.28)

2.31(0.13)

0.00(0.00)

0.00(0.00)

0.00(0.00)

0.00(0.00)

0.00(0.00)

0.00(0.00)

0.00(0.00)

0.00 0.00 0.00 0.00(0.00)

0.26(0.15)

0.31(0.04)

0.19(0.06)

2.47(0.04)

Mn 0.02(0.01)

0.03(0.00)

0.04(0.00)

0.02(0.01)

0.04(0.01)

0.05(0.00)

0.01(0.01)

0.01(0.00)

0.01(0.00)

0.02(0.01)

0.01(0.01)

0.00(0.00)

0.00(0.00)

0.00 0.00 0.00 0.00(0.00)

0.00(0.00)

0.00(0.00)

0.00(0.00)

0.04(0.01)

Ca 0.25(0.13)

1.57(0.11)

1.39(0.10)

0.15(0.05)

1.63(0.13)

1.26(0.07)

1.96(0.01)

1.96(0.01)

1.94(0.01)

1.96(0.01)

1.95(0.00)

0.99(0.00)

1.00(0.00)

0.00 0.00 0.00 0.00(0.00)

0.01(0.01)

0.01(0.00)

0.00(0.00)

0.00(0.00)

Na 1.85(0.07)

0.54(0.13)

0.74(0.15)

1.87(0.05)

0.43(0.17)

0.91(0.08)

0.00(0.00)

0.00(0.00)

0.00(0.00)

0.00(0.00)

0.00(0.00)

0.00(0.00)

0.00(0.00)

0.99 1.01 0.97 0.97(0.00)

0.04(0.01)

0.02(0.01)

0.05(0.01)

0.00(0.00)

K 0.00(0.00)

0.03(0.01)

0.04(0.01)

0.00(0.00)

0.03(0.01)

0.04(0.01)

0.00(0.00)

0.00(0.00)

0.00(0.00)

0.00(0.00)

0.00(0.00)

0.00(0.00)

0.00(0.00)

0.00 0.00 0.00 0.00(0.00)

0.79(0.02)

0.83(0.02)

0.81(0.01)

0.00(0.00)

Total 15.05(0.02)

15.09(0.08)

15.20(0.09)

15.08(0.04)

15.13(0.11)

15.33(0.05)

8.00(0.00)

8.00(0.00)

8.00(0.00)

8.00(0.00)

8.00(0.00)

3.00(0.00)

3.00(0.00)

5.00 5.00 5.00 5.00(0.00)

6.92(0.03)

6.94(0.05)

6.91(0.00)

10.87(0.00)

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Table 3. Average composition of major elements (ox. wt %) and of recalculated structural formulae of minerals composing mafic rocks from the Chah Gorbeh Complex (numbers in brackets are standarddeviations)

Amphibole Epidote Titanite Plagioclase Phengite Chlorite

I1309 I1311 I1309 I1311 I1309 I1309 I1311 I1309 I1311 I1309

Sample MineralMicrostructuralposition

Amp2Core

Amp2Int

Amp2Rim Amp1

Amp2Core

Amp2Int

Amp2Rim

Ep1Inc in

AbEp2Core

Ep2Int

Ep2Rim

Ep2Core

Ep2Int

Ep2Rim

Ttn2Core

Ttn2Rim

AbPorphCore

AbPorph

Int

AbPorphRim

AbPorphCore

AbPorphRim

Wm1Inc inAmp2

Wm2Core

Wm2Rim

Wm1Inc in

Ab Wm2 Chl

No. analyses 3 7 3 3 3 7 5 3 3 3 3 1 1 1 1 1 2 2 2 3 3 5 7 4 5 9 3

SiO2 55.91(0.18)

55.24(1.19)

56.24(0.58)

50.66(0.51)

55.29(1.01)

55.75(0.72)

54.95(1.01)

38.07(0.06)

37.21(0.04)

37.79(0.45)

37.92(0.34)

37.83 38.26 38.27 30.91 31.05 68.86(0.30)

69.08(0.24)

70.09(1.55)

69.29(0.51)

69.33(0.29)

51.69(0.12)

51.42(0.53)

51.21(0.45)

51.45(1.27)

50.78(0.46)

26.10(0.57)

TiO2 0.11(0.04)

0.12(0.09)

0.08(0.03)

0.06(0.05)

0.17(0.08)

0.13(0.06)

0.18(0.06)

0.10(0.03)

0.22(0.14)

0.25(0.04)

0.36(0.38)

0.10 0.09 0.03 35.71 35.66 0.03(0.04)

0.01(0.01)

0.04(0.04)

0.02(0.00)

0.02(0.02)

0.25(0.08)

0.24(0.07)

0.20(0.06)

0.24(0.09)

0.29(0.07)

0.07(0.03)

Al2O3 8.20(0.35)

8.58(0.49)

8.22(0.49)

4.58(0.63)

7.95(0.71)

8.15(0.55)

7.90(0.43)

24.43(0.30)

21.88(0.78)

23.64(0.69)

24.19(0.43)

23.76 22.49 23.14 2.62 2.60 19.86(0.38)

19.86(0.01)

20.26(0.23)

19.90(0.15)

19.94(0.15)

27.95(1.97)

28.46(0.53)

28.62(0.27)

27.00(1.01)

28.82(0.59)

20.40(0.44)

Cr2O3 0.02(0.02)

0.02(0.01)

0.02(0.04)

0.01(0.00)

0.01(0.02)

0.02(0.03)

0.01(0.01)

0.00(0.00)

0.00(0.00)

0.04(0.03)

0.03(0.03)

0.00 0.06 0.00 0.06 0.00 0.02(0.02)

0.01(0.01)

0.02(0.03)

0.03(0.03)

0.05(0.03)

0.03(0.02)

0.03(0.03)

0.02(0.02)

0.03(0.02)

0.02(0.02)

0.00(0.00)

FeO* 19.12(0.55)

19.76(0.69)

19.29(0.26)

19.44(0.33)

19.41(0.24)

19.35(0.55)

20.26(0.86)

12.09(0.20)

12.91(0.52)

12.89(0.39)

12.25(0.36)

12.73 13.73 13.76 0.66 0.70 0.09(0.05)

0.12(0.00)

0.15(0.04)

0.10(0.08)

0.21(0.08)

5.96(0.80)

5.53(0.36)

5.35(0.13)

6.08(1.55)

5.48(0.40)

25.75(0.06)

MgO 6.95(0.64)

6.41(0.32)

6.41(0.53)

10.83(0.37)

6.91(0.26)

6.94(0.28)

6.53(0.32)

0.02(0.01)

0.00(0.00)

0.01(0.01)

0.01(0.01)

0.00 0.01 0.02 0.00 0.01 0.00(0.00)

0.01(0.02)

0.01(0.00)

0.00(0.00)

0.01(0.01)

2.44(0.44)

2.27(0.27)

2.08(0.10)

2.52(0.20)

2.10(0.20)

15.30(0.35)

MnO 0.15(0.03)

0.15(0.07)

0.14(0.08)

0.18(0.04)

0.13(0.05)

0.13(0.05)

0.15(0.04)

0.11(0.04)

0.22(0.14)

0.25(0.04)

0.36(0.38)

0.50 0.19 0.19 0.10 0.08 0.00(0.00)

0.01(0.01)

0.05(0.02)

0.02(0.02)

0.02(0.04)

0.01(0.01)

0.00(0.01)

0.03(0.02)

0.01(0.02)

0.02(0.02)

0.28(0.02)

CaO 0.54(0.01)

0.68(0.38)

0.42(0.32)

9.02(0.96)

0.58(0.29)

0.58(0.20)

0.74(0.22)

23.04(0.14)

23.59(0.42)

23.12(0.03)

22.84(0.42)

22.64 23.24 23.25 28.52 28.20 0.06(0.03)

0.06(0.03)

0.08(0.04)

0.07(0.02)

0.04(0.01)

0.03(0.01)

0.02(0.03)

0.01(0.01)

0.06(0.05)

0.02(0.02)

0.06(0.04)

Na2O 7.19(0.14)

7.23(0.16)

7.35(0.13)

2.58(0.65)

7.17(0.15)

7.12(0.12)

7.11(0.09)

0.00(0.00)

0.05(0.02)

0.01(0.00)

0.01(0.01)

0.01 0.03 0.01 0.01 0.03 11.97(0.04)

12.13(0.33)

11.60(0.54)

12.19(0.16)

12.30(0.10)

0.33(0.22)

0.40(0.09)

0.46(0.04)

0.26(0.11)

0.41(0.08)

0.00(0.00)

K2O 0.01(0.01)

0.02(0.01)

0.02(0.00)

0.31(0.07)

0.02(0.01)

0.01(0.01)

0.03(0.02)

0.01(0.01)

0.01(0.01)

0.01(0.00)

0.00(0.00)

0.00 0.00 0.00 0.01 0.00 0.02(0.01)

0.02(0.00)

0.03(0.02)

0.02(0.01)

0.04(0.01)

9.88(0.37)

9.64(0.11)

9.61(0.04)

9.86(0.28)

9.72(0.21)

0.00(0.00)

Total 98.21(0.62)

98.19(0.33)

98.18(0.38)

97.68(0.35)

97.66(0.43)

98.19(0.56)

97.86(0.36)

97.87(0.06)

96.09(1.51)

98.01(0.78)

97.97(0.12)

97.57 98.11 98.68 98.60 98.33 100.91(0.80)

101.30(0.08)

102.31(1.14)

101.63(0.34)

101.97(0.53)

98.58(0.55)

98.03(0.65)

97.56(0.43)

97.51(0.62)

97.67(0.52)

87.97(0.74)

Si 7.93(0.02)

7.87(0.11)

7.98(0.04)

7.47(0.05)

7.91(0.11)

7.92(0.05)

7.88(0.08)

2.99(0.00)

2.99(0.05)

2.97(0.01)

2.98(0.02)

2.99 3.02 3.00 1.01 1.01 2.8(0.01)

2.97(0.02)

3.00(0.05)

2.97(0.02)

2.96(0.01)

3.37(0.04)

3.36(0.02)

3.36(0.02)

3.40(0.03)

3.34(0.02)

2.75(0.04)

Ti 0.01(0.00)

0.01(0.01)

0.01(0.00)

0.01(0.01)

0.02(0.01)

0.01(0.01)

0.02(0.01)

0.01(0.00)

0.01(0.01)

0.01(0.00)

0.02(0.02)

0.01 0.01 0.00 0.87 0.88 0.00(0.00)

0.00(0.00)

0.00(0.00)

0.00(0.00)

0.00(0.00)

0.01(0.00)

0.01(0.00)

0.01(0.00)

0.01(0.00)

0.01(0.00)

0.00(0.00)

Al 1.37(0.05)

1.44(0.08)

1.37(0.08)

0.80(0.11)

1.34(0.12)

1.36(0.09)

1.34(0.07)

2.26(0.03)

2.07(0.04)

2.19(0.04)

2.24(0.03)

2.21 2.09 2.13 0.10 0.10 1.01(0.01)

1.01(0.00)

1.02(0.01)

1.01(0.01)

1.00(0.00)

2.15(0.13)

2.19(0.04)

2.22(0.02)

2.10(0.04)

2.23(0.04)

2.54(0.07)

Cr 0.00(0.00)

0.00(0.00)

0.00(0.00)

0.00(0.00)

0.00(0.00)

0.00(0.00)

0.00(0.00)

0.00(0.00)

0.00(0.00)

0.00(0.00)

0.00(0.00)

0.00 0.00 0.00 0.00 0.00 0.00(0.00)

0.00(0.00)

0.00(0.00)

0.00(0.00)

0.00(0.00)

0.00(0.00)

0.00(0.00)

0.00(0.00)

0.00(0.00)

0.00(0.00)

0.00(0.00)

Fe* 2.27(0.06)

2.35(0.10)

2.29(0.04)

2.40(0.04)

2.32(0.04)

2.30(0.07)

2.43(0.12)

0.79(0.01)

0.87(0.02)

0.85(0.03)

0.81(0.03)

0.84 0.90 0.90 0.02 0.02 0.00(0.00)

0.00(0.00)

0.01(0.00)

0.00(0.00)

0.01(0.00)

0.33(0.05)

0.30(0.02)

0.29(0.01)

0.34(0.09)

0.30(0.02)

2.27(0.02)

Mg 1.47(0.14)

1.36(0.06)

1.35(0.12)

2.38(0.09)

1.47(0.06)

1.47(0.07)

1.40(0.07)

0.00(0.00)

0.00(0.00)

0.00(0.00)

0.00(0.00)

0.00 0.00 0.00 0.00 0.00 0.00(0.00)

0.00(0.00)

0.00(0.00)

0.00(0.00)

0.00(0.00)

0.24(0.05)

0.22(0.03)

0.20(0.01)

0.25(0.02)

0.21(0.02)

2.40(0.03)

Mn 0.02(0.00)

0.02(0.01)

0.02(0.01)

0.02(0.01)

0.02(0.01)

0.02(0.01)

0.02(0.00)

0.01(0.00)

0.02(0.01)

0.02(0.00)

0.02(0.03)

0.03 0.01 0.01 0.00 0.00 0.00(0.00)

0.00(0.00)

0.00(0.00)

0.00(0.00)

0.00(0.00)

0.00(0.00)

0.00(0.00)

0.00(0.00)

0.00(0.00)

0.00(0.00)

0.02(0.01)

Ca 0.08(0.00)

0.10(0.06)

0.06(0.05)

1.43(0.16)

0.09(0.04)

0.09(0.03)

0.11(0.04)

1.94(0.01)

2.03(0.00)

1.95(0.02)

1.92(0.03)

1.92 1.96 1.95 1.00 0.99 0.00(0.00)

0.00(0.00)

0.00(0.00)

0.00(0.00)

0.00(0.00)

0.00(0.00)

0.00(0.00)

0.00(0.00)

0.00(0.00)

0.00(0.00)

0.01(0.01)

Na 1.98(0.03)

2.00(0.03)

2.02(0.03)

0.74(0.18)

1.99(0.03)

1.96(0.03)

1.98(0.02)

0.00(0.00)

0.01(0.00)

0.00(0.00)

0.00(0.00)

0.00 0.00 0.00 0.00 0.00 1.00(0.01)

1.01(0.02)

0.96(0.05)

1.01(0.01)

1.02(0.01)

0.04(0.03)

0.05(0.01)

0.06(0.00)

0.03(0.01)

0.05(0.01)

0.00(0.00)

K 0.00(0.00)

0.00(0.00)

0.00(0.00)

0.06(0.01)

0.00(0.00)

0.00(0.00)

0.01(0.00)

0.00(0.00)

0.00(0.00)

0.00(0.00)

0.00(0.00)

0.00 0.00 0.00 0.00 0.00 0.00(0.00)

0.00(0.00)

0.00(0.00)

0.00(0.00)

0.00(0.00)

0.82(0.04)

0.80(0.01)

0.80(0.00)

0.83(0.03)

0.82(0.02)

0.00(0.00)

Total 15.13(0.02)

15.16(0.17)

15.11(0.07)

15.29(0.03)

15.16(0.04)

15.14(0.06)

15.18(0.08)

8.00(0.00)

8.00(0.00)

8.00(0.00)

8.00(0.00)

8.00 8.00 8.00 3.00 3.00 5.00(0.00)

5.00(0.00)

5.00(0.00)

5.00(0.00)

5.00(0.00)

6.97(0.03)

6.95(0.01)

6.95(0.01)

6.97(0.05)

6.96(0.02)

10.86(0.01)

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Blueschists

oftheA

narakM

etamorphic

Com

plex(Iran)

327

Table 4. Average composition of major elements (ox. wt %) and of recalculated structural formulae of minerals composing mafic rocks from the Ophiolitic Complex (numbers in brackets are standarddeviations)

Amphibole Titanite Plagioclase Phengite

AK33A AK36 AK33B AK36 AK33A AK36 AK33A AK36

Sample MineralMicrostructuralposition Amp2 fine Amp2 Core Amp2 Rim Amp2 fine Amp2 Core Amp2 Rim Ttn2 Ttn2 Ab Porph Ab Porph Wm2 Wm2No. analyses 3 3 2 3 3 6 3 5 5 12 5 3

SiO2 55.20 (0.24) 55.07 (0.20) 55.47 (0.18) 55.60 (0.35) 55.74 (0.32) 55.74 (0.46) 30.78 (0.31) 30.73 (0.24) 68.82 (0.28) 68.87 (0.30) 53.56 (0.60) 53.68 (0.87)TiO2 0.07 (0.01) 0.04 (0.04) 0.03 (0.03) 0.14 (0.16) 0.06 (0.01) 0.03 (0.04) 36.39 (1.38) 36.68 (1.24) 0.01 (0.02) 0.01 (0.01) 0.08 (0.05) 0.11 (0.02)Al2O3 2.36 (0.79) 2.29 (0.08) 2.31 (0.30) 2.38 (0.66) 2.59 (0.42) 2.10 (0.46) 1.74 (0.63) 1.44 (0.60) 19.56 (0.14) 19.64 (0.20) 24.93 (0.85) 24.57 (0.88)Cr2O3 0.06 (0.04) 0.06 (0.05) 0.05 (0.03) 0.03 (0.04) 0.07 (0.04) 0.05 (0.04) 0.01 (0.01) 0.04 (0.05) 0.00 (0.00) 0.01 (0.01) 0.11 (0.06) 0.15 (0.03)FeO* 19.10 (0.43) 20.73 (1.17) 20.64 (0.55) 19.20 (0.12) 19.75 (0.43) 19.21 (1.56) 1.09 (0.34) 1.13 (0.14) 0.11 (0.05) 0.06 (0.04) 5.14 (0.70) 4.78 (1.33)MgO 10.92 (0.66) 9.79 (0.87) 9.73 (0.03) 10.43 (0.59) 10.01 (0.27) 10.67 (0.74) 0.04 (0.03) 0.04 (0.02) 0.01 (0.01) 0.01 (0.01) 3.50 (0.07) 3.73 (0.07)MnO 0.15 (0.00) 0.15 (0.03) 0.12 (0.01) 0.15 (0.02) 0.12 (0.01) 0.15 (0.04) 0.02 (0.02) 0.01 (0.01) 0.01 (0.02) 0.02 (0.02) 0.01 (0.02) 0.00 (0.00)CaO 3.47 (1.58) 2.26 (0.68) 1.64 (0.18) 2.01 (0.78) 1.12 (0.20) 2.31 (1.83) 28.06 (0.21) 28.07 (0.28) 0.01 (0.01) 0.01 (0.01) 0.04 (0.03) 0.01 (0.01)Na2O 5.36 (0.77) 6.23 (0.33) 6.42 (0.02) 6.44 (0.54) 6.68 (0.05) 6.06 (0.95) 0.02 (0.02) 0.05 (0.06) 11.54 (0.10) 11.57 (0.09) 0.11 (0.04) 0.11 (0.05)K2O 0.08 (0.05) 0.05 (0.02) 0.04 (0.01) 0.03 (0.00) 0.02 (0.01) 0.03 (0.01) 0.09 (0.06) 0.07 (0.10) 0.03 (0.01) 0.00 (0.01) 9.79 (0.53) 9.79 (0.17)Total 96.78 (0.20) 96.67 (0.53) 96.43 (0.32) 96.42 (0.20) 96.18 (0.38) 96.35 (0.32) 98.25 (0.28) 98.25 (0.42) 100.10 (0.46) 100.19 (0.47) 97.26 (0.58) 96.93 (0.51)

Si 7.89 (0.02) 7.91 (0.05) 7.97 (0.01) 7.96 (0.06) 7.99 (0.03) 7.98 (0.04) 1.01 (0.01) 1.01 (0.01) 3.01 (0.01) 3.01 (0.01) 3.52 (0.02) 3.53 (0.02)Ti 0.01 (0.00) 0.00 (0.00) 0.00 (0.00) 0.01 (0.02) 0.01 (0.00) 0.00 (0.00) 0.90 (0.03) 0.91 (0.03) 0.00 (0.00) 0.00 (0.00) 0.00 (0.00) 0.01 (0.00)Al 0.40 (0.13) 0.39 (0.01) 0.39 (0.05) 0.40 (0.11) 0.44 (0.07) 0.35 (0.08) 0.07 (0.02) 0.06 (0.02) 1.01 (0.00) 1.01 (0.01) 1.93 (0.05) 1.91 (0.05)Cr 0.01 (0.00) 0.01 (0.01) 0.01 (0.00) 0.00 (0.00) 0.01 (0.00) 0.01 (0.00) 0.00 (0.00) 0.00 (0.00) 0.00 (0.00) 0.00 (0.00) 0.01 (0.00) 0.01 (0.00)Fe* 2.28 (0.05) 2.49 (0.15) 2.48 (0.06) 2.30 (0.02) 2.37 (0.06) 2.30 (0.19) 0.03 (0.01) 0.03 (0.00) 0.00 (0.00) 0.00 (0.00) 0.28 (0.04) 0.26 (0.08)Mg 2.33 (0.14) 2.10 (0.17) 2.08 (0.00) 2.23 (0.13) 2.14 (0.05) 2.28 (0.15) 0.00 (0.00) 0.00 (0.00) 0.00 (0.00) 0.00 (0.00) 0.34 (0.01) 0.37 (0.01)Mn 0.02 (0.00) 0.02 (0.00) 0.02 (0.00) 0.02 (0.00) 0.01 (0.00) 0.02 (0.00) 0.00 (0.00) 0.00 (0.00) 0.00 (0.00) 0.00 (0.00) 0.00 (0.00) 0.00 (0.00)Ca 0.53 (0.24) 0.35 (0.10) 0.25 (0.03) 0.31 (0.12) 0.17 (0.03) 0.35 (0.28) 0.99 (0.01) 0.99 (0.01) 0.00 (0.00) 0.00 (0.00) 0.00 (0.00) 0.00 (0.00)Na 1.49 (0.21) 1.74 (0.10) 1.79 (0.01) 1.79 (0.15) 1.86 (0.01) 1.68 (0.27) 0.00 (0.00) 0.00 (0.00) 0.98 (0.01) 0.98 (0.01) 0.01 (0.01) 0.01 (0.01)K 0.01 (0.01) 0.01 (0.00) 0.01 (0.00) 0.01 (0.00) 0.00 (0.00) 0.01 (0.00) 0.00 (0.00) 0.00 (0.00) 0.00 (0.00) 0.00 (0.00) 0.82 (0.05) 0.82 (0.01)Total 14.96 (0.02) 15.01 (0.03) 14.99 (0.01) 15.03 (0.03) 15.00 (0.01) 14.99 (0.06) 3.00 (0.00) 3.00 (0.00) 5.00 (0.00) 5.00 (0.00) 6.92 (0.05) 6.92 (0.03)

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respect to white micas of mafic rocks from the CGCand MC. As already discussed by Zanchi et al. (2015),this comparison suggests that phengitic micas of meta-pillow basalts likely re-equilibrated with riebeckite un-der blueschist-facies conditions during a prograde, re-latively HP, metamorphism.

5.b.4. Thermodynamic modelling

A P–T isochemical section was calculated withPERPLE_X software (Connolly, 2005), consideringthe bulk composition of sample AK14b and H2O-saturated conditions, using the thermodynamic data-base and equation of state for H2O of Holland &Powell (1998, upgraded 2002). No phase has been ex-cluded from the calculation. Solution models of Hol-land & Powell (1998, 2003) were used for olivine,orthopyroxene, clinopyroxene, garnet, phengite, feld-spar, epidote, chlorite and biotite. Two different solu-tion models have been used to model Ca-amphiboleand Na-amphibole from Dale et al. (2005), which re-quire Fe2O3 and FeO concentrations. Fe+3-rich miner-als such as epidote also occurs in both HP–LT and syn-S2 metamorphic event, and the use of a solid solutionmodel for epidote is crucial to model their possible sta-bility fields. For this reason, we measured the FeO andFe2O3 concentration of the whole rock by chemical ti-tration and considered FeO and Fe2O3 as distinct chem-ical components in the thermodynamic forward mod-elling. The phase diagram section reported in Figure 7shows that the (pre-S2) Cpx-free mineral associationNa-Amp1 + Wm1 + Ttn1 + Ep1 + Pl1 is stable in a nar-row P–T range between 0.8 and 0.95 GPa and 410–460 °C, respectively. The appearance of chlorite inthe S2 Cpx-free and Bio-free mineral assemblage(blue text in Fig. 7) Amp2 (core) + Chl + Wm2 + Ep2(core) + Pl2 occurs at similar temperature conditionsand lower pressures, down to 0.65 GPa.

Comparing the Morghab metamorphic peak condi-tions with those of the Chah Gorbeh and Ophioliteunits (0.75–0.85 GPa and 350–400 °C) reported byZanchi et al. (2015), the MC mafic schists experiencedHP metamorphism before the development of the S2foliation as will be discussed later. It is worth notingthat the thermodynamic modelling results are stronglysupported by the microstructural evidence (Fig. 5).

Thermodynamic modelling of the stability of solidsolutions with Fe3+-bearing end-member minerals(i.e. epidote and Na-amphibole) requires the assump-tion that the bulk Fe2+/Fe3+ ratio was constant dur-ing the entire metamorphic cycle of the rock. Wetherefore compared the P–T isochemical section ofFigure 7 with a T–fO2 isochemical section calculatedat 0.9 GPa, which is portrayed in Figure 8. The startingcomposition is the same as that used in Figure 7, butconsidering FeO as total iron. The peak (pre-S2) Na-Amp1 + Wm1 + Ttn1 + Ep1 + Pl1 mineral associationis stable at 460 °C and logfO2 ∼= −25.5, correspondingto FMQ-1 (FMQ is the reference fayalite–magnetite–quartz buffer). It is worth noting that the stability of

epidote is strongly influenced by the oxidation state ofthe bulk composition, therefore by the Fe2+/Fe3+ ra-tio of the rock. The fO2 value calculated from forwardmodelling enables a back calculation of the expectedFeO and Fe2O3 content of the whole rock using theequation of Kress & Carmichael (1988). The measuredFe2+/Fe3+ ratio at T = 450 °C and P = 1 atm would bestable at logfO2 = −24, which is perfectly consistentwith the values calculated in Figure 8, if we considerthat fO2 slightly decreases with increasing pressure.We can therefore assume that the Fe2+/Fe3+ contentused to reconstruct the P–T stability field of epidote-and Na-amphibole-bearing assemblage is representat-ive of the composition of the rock at the pressure-peakconditions.

6. Discussion

The three main units constituting the gross part of theAMC show a complex structural pattern that is the res-ult of at least three deformation events. The MC, CGCand OC were refolded together during the last deform-ation stage (D3) and overthrust by the Lakh Marbleunit (Zanchi et al. 2015). D3 structure developmentsoccurred at non- or very low-grade metamorphic con-ditions (Fig. 3e). The metamorphic evolution of thethree units should then be related to pre-D3 deform-ation stages.

Field structural analyses (Zanchi et al. 2015) indi-viduated a regional foliation (S2) that is almost parallelin the MC and CGC units, suggesting that it is probablyrelated to the same deformation event that affected thetwo units during or after their tectonic coupling. How-ever, the detailed study of microstructures and mineralchemistry of blueschist-facies mineral assemblages inthe AMC rocks presented in this study suggests thatthe three units experienced tectonometamorphic evol-utions that differed in some stages.

The spatial relationships between compositionallydistinct amphiboles in the MC, CGC and OC maficrocks point to a pre-S2 (regional foliation) pres-sure peak in the MC. Amphiboles (Amp1) includedin plagioclase (Pl1) and epidote (Ep2) porphyro-blasts are glaucophane or ferroglaucophane (Table 2;Fig. 6), whereas Amp2, aligned along the main foli-ation (S2), are calcic amphiboles (Amp2 in Table 2).Deformation–recrystallization relationships thus in-dicate that peak pressure (0.80–0.95 GPa at T of 410–450 °C, Fig. 7) was reached by the MC mafic schistsbefore the development of the S2 foliation. Calcicamphiboles (Amp2) and epidotes (Ep2) syn-kinematicwith the S2 foliation display in turn a core–rim com-positional zoning indicative of a pressure increase dur-ing or after the D2 deformation event. Based on thesedata we can define a clockwise P–T path for the AMC(Fig. 9), with a blueschist-facies stage followed bygreenschist re-equilibration at lower pressure and laterunderplating to greater depths. The amphibole chem-ical composition suggests that the late-S2 metamorphic




Figure 7. (Colour online) Isochemical P–T section for Morghab meta-basalt calculated in the system SiO2 (50.27) – Al2O3 (14.42) –MgO (5.61) – CaO (7.27) – Na2O (4.53) – K2O (0.82) – TiO2 (1.41) – FeO (8.57) – Fe2O3 (3.43). Values in brackets are oxide wt. %.The red area shows the hypothetical stability field of the relict HP paragenesis preserved in porphyroblastic epidote, as evidenced bymicrostructural and mineral chemistry analyses. Abbreviations for solid solution models: Amph – Ca-amphiboles and Na-amphiboles;Bio – biotite; Chl – chlorite; Ep – epidote; Cpx – clinopyroxene; Gt – garnet; Kfs – alkali feldspar; Mica – phengite; Pl – ternaryfeldspar. Abbreviations for phases: ab – albite; acti – actinolite; fctd – Fe-chloritoid; ilm – ilmenite; law – lawsonite; mic – microcline;mt – magnetite; q – quartz; pa – paragonite; pre – prehnite; pump – pumpellyite; ru – rutile; sph – titanite (sphene); stlp – stilpnomelane;vsv – vesuvianite; zo – zoisite. The paragenesis at pressure peak observed in thin-section is highlighted in red. The same equilibriumphase assemblage with chlorite in addition (blue text in figure) occurs at the same T, but a lower pressure with respect to the chlorite-free one.

stage did not reach the pressure-peak experienced bythe MC mafic rocks during the pre-D2 evolution.

The CGC and OC mafic rocks display a contrast-ing metamorphic evolution. HP–LT metamorphism isdemonstrated to have followed a greenschist-faciesmetamorphic event as shown by microstructures andmineral chemistry. Sodic amphibole (Amp2) in CGCrocks is aligned along the main foliation (S2, Fig. 5),whereas low-P actinolitic amphibole (Amp1) is pre-served as inclusions within Pl1 and Ep1 porphyroblastswhich are pre-kinematic with respect to S2. The com-positional variation of epidote, with syn-S2 Ep2 en-riched in Fe3+ with respect to Ep1, and the growth of

rutile instead of titanite as the main Ti-bearing phase,could be considered as clues that point to a P increaseduring the D2 deformation event. Nevertheless, specialcare should be taken as at low to medium pressure,the compositional variation of epidote also depends onfO2, coexisting Ca-Al silicates and H2O-content (e.g.Grapes & Hoskin, 2004).

The meta-pillow basalts provide less informationon the deformation–metamorphism relationships asthey are substantially non-deformed. A poorly de-veloped primary foliation (S1) is defined by the SPO ofgreenschist-facies minerals, with actinolitic amphibole(Amp1) and chlorite as the main mineral phases. The




Figure 8. (Colour online) Isochemical T–fO2 section for Morghab meta-basalt calculated in the same system as that of Figure 7.The tiny red area shows the hypothetical stability field of the HP paragenesis. The red curve delineates the stability of epidote solidsolution as a function of the oxidation state of the rock. The grey curve is the FMQ (fayalite–magnetite–quartz) reference redox buffer.Abbreviations are the same as Figure 7, except hem – haematite.

S1 fabric is overprinted by the static growth of sodicamphibole (Amp2), white mica (Wm2) with Si up to3.56 apfu and a second generation of titanite (Ttn2).The reconstructed recrystallization stages point to aP–T evolution similar to the one shown by the CGC,with an early greenschist-facies stage followed by re-equilibration at higher pressure (Fig. 9).

Isochemical P–T pseudosections (Fig. 7) presentedin this work and in Zanchi et al. (2015) suggest thatthe CGC and meta-basalts of the OC reached a sim-ilar pressure (0.6–0.9 GPa) at temperatures slightlyhigher for the OC (390–440 °C) with respect to theCGC (320–380 °C). Based on these considerations wecan argue that the CGC mafic rocks and the OCmeta-basalts experienced counter-clockwise P–T pathswith a first metamorphic stage that occurred at rel-atively HT and LP (greenschist facies), followed by

a HP–LT (epidote blueschist facies) re-equilibration(Fig. 9).

The metamorphic gap between syn-S2 foliationphase assemblage of the MC (greenschist facies) andCGC (blueschist facies) indicates that the regional fo-liation in the two tectonometamorphic units formed atdifferent ambient conditions.

The detailed microstructural and microprobe chem-ical analyses reveal different metamorphic stagescharacterizing the evolution of each AMC unit. Agreenschist-facies metamorphism, likely occurring atlow pressure as suggested by the absence of garnet andNa-clinopyroxene, has been individuated in the CGCand OC rocks. Such a stage is recorded by the oc-currence of calcic amphiboles included in pre-mainfoliation porphyroblasts (CGC) or as the primary as-semblage (OC) overprinted by the later static growth




Figure 9. (Colour online) (a) P–T estimates and inferred P–T paths of the CGC, MC and OC blueschists. P–T estimates for theGCG and OC metabasites are from Zanchi et al. (2015). Representative reconstructed P–T paths for blueschists from the Franciscansubduction complex (Ernst, 1988; Banno et al. 2000; Ukar & Cloos, 2014), Oman (Yamato et al. 2007; Agard et al. 2010) and Zagros(Angiboust et al. 2016) are reported for comparison. Metamorphic facies are from Evans (1990). (b) Cartoon depicting the suggestgeodynamic scenario in which the tectonic units of the AMC acquired their contrasting P–T evolutionary paths.

of HP-phase assemblages. This stage could be relatedto an early heating during subduction initiation, beforethe depression of the geotherms by continuous subduc-tion of the oceanic crust. An alternative hypothesis isthat this low-pressure stage is related to seafloor meta-morphism overprinted during subduction. Ocean floormetamorphism is commonly associated with diffuseveining and intense metasomatism, which are both ab-

sent in the AMC units. Quartz- and calcite-bearingveins occur in the pillow meta-basalts (Fig. 4d), butthey cross-cut both pillows and sheared domains with ablueschist-facies overprint (Fig. 4b). This suggests thatveining represents a late-stage feature, post-dating theburial and exhumation of the OC unit. The same holdstrue also for the mafic schist and associated rocks ofthe CGC (Zanchi et al. 2015).




The counter-clockwise P–T paths experienced byCGC and OC were likely due to early heating whenthey were underthrust beneath the southern Eurasiamargin at the beginning of subduction (Fig. 9).

Such counter-clockwise P–T paths are commonwithin evolved accretionary wedges, when mafic rocksderived from the subducting oceanic crust or sedi-ments scraped off the evolving prism are accreted inthe early stages of subduction and later buried to in-creasing depth while geotherms are progressively de-pressed (Cloos, 1982).

On the other hand, the P–T path of the MC is ratherpeculiar: an early HP–LT metamorphic stage is fol-lowed by greenschist-facies re-equilibration with a fur-ther pressure increase as suggested by amphibole andepidote compositional zoning. P–T estimates made byforward thermodynamic modelling (fig. 24 in Zanchiet al. 2015) indicate similar pressures of the CGC andOC relative to the pre-S2 metamorphic peak, but athigher T (Fig. 9). Dynamic metamorphism represen-ted by the syn-S2 metamorphic stage re-equilibratedthe HP–LT phase assemblages at greenschist-faciesconditions. The chemical compositions (Fig. 6) of am-phibole and epidote that grew during this greenschist-facies event point to a late- to post-deformation pres-sure increase. Summarizing, (Table 2) the MC maficrocks experienced a clockwise P–T path, with anearly HP–LT stage followed by exhumation at lowerdepths (greenschist-facies re-equilibration) and sub-sequent pressure increase likely due to re-subductionwithin the accretionary wedge.

Despite the possible difference in P–T paths experi-enced by the three units, a temperature/depth gradientof c. 15 °C km−1 seems to be the best estimate for thepeak pressure in all units (Fig. 9).

Counter-clockwise P–T paths within exhumed ac-cretionary prisms are well known for block-in-matrixsubduction mélanges, like the Franciscan Complex(Ukar, 2012; Ukar & Cloos, 2014), but are less com-mon in HP–LT complexes made of coherent tectonicunits. The Hajiabad blueschists of the Zagros oro-gen (Agard et al. 2006; Angiboust et al. 2016) dis-play instead such features (Fig. 9). Despite differentmetamorphic peak conditions, several units underwentan initial heating stage during underplating, followedby HP–LT dynamic metamorphism. Geochronologicaldata suggest that tectonic units reached pressure peakat different times (Angiboust et al. 2016). The oc-currence and preservation of counter-clockwise P–Tpaths are likely favoured for tectonic units that wereunderplated in the first subduction stages, along arelatively hot (15–17 °C km−1) geothermal gradient.Geotherms are then progressively depressed as thesubduction zone evolves and early subducted units canbe exhumed along a cold (< 10 °C km−1) subductionchannel.

The age of HP–LT metamorphism in the AMC unitsis debated. Bagheri & Stampfli (2008) proposed anEarly Permian age (285 Ma) for blueschists of theCGC, and a ‘Variscan’ age for the HP–LT metamorph-

ism of the MC, suggesting that the two units formedin distinct times and palaeotectonic settings. However,these geochronological data are highly debatable (Zan-chi et al. 2015) as microstructural and chemical fea-tures of the dated minerals are loosely constrained.To our knowledge metamorphism and deformation ofAMC units should be constrained to pre-Sakmarian (c.300 Ma or older) times (Zanchi et al. 2015) and no ro-bust data exist on the age of HP–LT metamorphism.Even if we take as valid the available geochronologicaldata of the HP–LT metamorphism of the AMC, weneed to consider that in the context of a long-lastingsubduction (e.g. several tens of millions of years) thesubducted rocks could reach the metamorphic peakat different ambient conditions and at different times(e.g. Gerya, Stockhert & Perchuk, 2002; Ukar, 2012).Therefore, the meta-basalts and the blueschists of theCGC and MC, even if they recorded contrasting P–Tevolutions and were possibly metamorphosed at differ-ent ages, could still be considered as parts of a singleaccretionary complex.

The nature and provenance of the continental blocksthat collided with the southern Eurasian margin is alsocontroversial. The Anarak block strongly differs fromthe Upper Palaeozoic successions of Central and NorthIran that record an extensional evolution related tothe opening of the Neo-Tethys that caused the north-ward drift of Iran (Angiolini et al. 2007; Gaetani et al.2009). A possible provenance, discussed in Zanchiet al. (2015), is that the AMC was part of the ac-cretionary wedge developed above the Palaeo-Tethyssubduction zone. In NE Iran, the Variscan–Cimmeriancomplexes of Binalood and Fariman (Sheikholeslami& Kouhpeyma, 2012; Zanchetta et al. 2013) recordedthe upper plate history of the Palaeo-Tethys subduc-tion. The AMC could be originally placed to the SWof the Fariman complex and later shifted westwards toits present-day position in the framework of the largecounter-clockwise rotation along a vertical axis that af-fected Central Iran from Triassic time (Muttoni et al.2009; Mattei et al. 2015).

7. Conclusions

The AMC of Central Iran is made of several unitsamong which three of them, the Morghab, ChahGorbeh and meta-basalts complexes, have been meta-morphosed at HP–LT conditions within the blueschistfacies.

At least three deformation events have been recog-nized in the MC and CGC. The second deformationstage is responsible for the development of the re-gional foliation (S2) that is generally concordant inthe two units. Microstructural investigations and mi-croprobe analyses demonstrated that the main foliationdeveloped at blueschist-facies and greenschist-faciesconditions for the CGC and MC units, respectively.The main fabric element of the CGC and MC thusformed when the two units were at different depthswithin the accretionary wedge.




The reconstruction of the deformation–metamorphism relationships and P–T estimates ofthe metamorphic stages allowed the definition ofa clockwise P–T path for the MC and a counter-clockwise path for CGC and meta-basalts of the OC.P–T conditions at metamorphic peak were of 410–450 °C at 0.78–0.9 GPa for the MC, 390–440 °C at0.6–0.9 GPa for the meta-basalts and 320–380 °C at0.6–0.9 GPa for the CGC.

The presented data suggest that the three unitsreached almost the same depth (c. 25 km) within theAnarak accretionary wedge, but followed differentsubduction–exhumation paths that resulted in a con-trasting P–T evolution.

Acknowledgements. This paper results from the PRIN2010/2011 Project BIRTH AND DEATH OF OCEANICBASINS: GEODYNAMIC PROCESSES FROM RIFTINGTO CONTINENTAL COLLISION IN MEDITERRANEANAND CIRCUM-MEDITERRANEAN OROGENS and theDARIUS PROGRAMME. We are grateful to Vachik Hair-apetian (Department of Geology, Esfahan Branch, IslamicAzad University, Iran) for providing us information on theage of the Lakh Marble. Comments and suggestions by YannRolland and an anonymous reviewer helped us to improvethe quality of the paper. G. Capponi is warmly thanked forthe editorial handling of the paper. We are grateful to theGeological Survey of Iran (Teheran branch) for its continu-ous support during several years of common research activ-ities in Central Iran.

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